There are numerous well known dewatering systems in use today in the papermaking industry. A common system is a Fourdrinier paper machine employing a flat box section with a plurality of serially arranged flat boxes interconnected by a header. At least the last box is usually identified as a dry box. It is common practice to interconnect the boxes as a series with vacuum controllers maintaining the desired vacuum conditions. In the interconnected system, a maximum vacuum level is generally required at the header due to the pressure drop across the controller for the last flat box or boxes which normally run at the highest vacuum. The need for the maximum vacuum level at the header produces high energy demands even when the vacuum level is not necessarily required for dewatering purposes in the system. Accordingly, a system which reduces this energy consumption condition in a papermaking machine would be extremely desirable.
There are several basic types of vacuum pumps used for providing the most efficient vacuum system for different types of papermaking machine applications such as the one discussed above. Three basic types of vacuum pumps used in the paper industry are the liquid ring pump, positive displacement pump, and the centrifugal exhauster, some times called a blower. Each type has its advantages and disadvantages with respect to the others and the choice of vacuum pump is based on system parameters. It is clearly desirable to be able to provide for the use of any type of vacuum pump mentioned above in a conventional manner as part of a papermaking machine such as a Fourdrinier paper machine. Furthermore, such use in conjunction with particular parts of the machinery such as the flat box section to provide a vacuum control system which produces significant energy savings so that a more efficient and inexpensive system results therefrom is a distinct improvement.
Cost savings from an energy standpoint have been realized in the design of systems which minimize horsepower requirements for vacuum pumps. These developments are amply disclosed in U.S. Pat. Nos. 4,308,077 and 4,329,201.
Variable speed motors also appear in the general state of the art in controlling vacuum systems. Examples of references of interest in this area are U.S. Pat. Nos. 3,490,689; 3,005,490; 3,935,061; and 3,077,924. Also, German Patent No. 2,849,881 and the Lavigne publication entitled Instrumentation Applications of the Foxboro Company in 1979 pages 208-209 are of interest in this respect. These systems are somewhat limited in that they relate only to a system of continuous adjustment in response to continuously changing vacuum conditions.
None of these systems recognizes the beneficial results that can be obtained by maintaining a constant high pump speed until a predetermined maximum vacuum level is reached and thereafter taking advantage of adjustable controls to actuate a variable speed motor to adjust the pump speed from that point on to monitor the vacuum level and maintaining the predetermined maximum vacuum level. This concept requires the introduction of a dual control system with one of the controls operating to run the variable speed motor and accordingly the pump at a constant speed until a set point is reached corresponding to the maximum vacuum level to initiate the operation of a second control to operate the variable speed motor as a variable speed control for the pump to regulate and constantly adjust the speed of the pump and the vacuum level in the system so that the maximum vacuum level is maintained thereafter.